Similar to bug 18245 for sqrtl, and with the same underlying cause, fmodl (-1.1L, 0.0L) and similar cases with (Inf, y) and with remainderl, produce spurious "inexact" exceptions with NaN results (these being fully determined functions, they should not do so, whereas for most functions such exceptions are OK).

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https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=8b1bab5ffa235bc494c33fdbe4c9994202936cfc
commit 8b1bab5ffa235bc494c33fdbe4c9994202936cfc
Author: Joseph Myers <joseph@codesourcery.com>
Date: Tue Jun 23 17:26:46 2015 +0000
Fix spurious "inexact" exceptions from __kernel_standard_l (bug 18245, bug 18583).
__kernel_standard_l converts long double arguments to double for use
in SVID "struct exception". This has special-case handling for when
that conversion would overflow or underflow but the original long
double function wouldn't. However, it turns out that "inexact"
exceptions can be spurious here as well, when the function is exactly
determined and __kernel_standard_l is being called for a domain error.
This patch fixes this by using feholdexcept / fesetenv to avoid
exceptions from the conversion, replacing the previous special-case
logic for overflow and underflow (this covers all functions using
__kernel_standard_l, not just those that actually need a change, since
there doesn't seem to be much point in restricting things just to the
functions that mustn't get "inexact" here).
Tested for x86_64 and x86.
[BZ #18245]
[BZ #18583]
* sysdeps/ieee754/k_standardl.c: Include <fenv.h>.
(__kernel_standard_l): Use feholdexcept and fesetenv around
conversion to double instead of special-casing overflow and
underflow.
* math/libm-test.inc (fmod_test_data): Add more tests.
(remainder_test_data): Likewise.
(sqrt_test_data): Likewise.
-----------------------------------------------------------------------
Summary of changes:
ChangeLog | 12 ++++++++++++
NEWS | 12 ++++++------
math/libm-test.inc | 9 +++++++++
sysdeps/ieee754/k_standardl.c | 34 +++++++++-------------------------
4 files changed, 36 insertions(+), 31 deletions(-)